1. Field of the Invention
The present invention is directed toward a method for controlling a vehicle heating, ventilation, and air conditioning (HVAC) system. In particular, the present invention relates to automatically sensing fogging conditions, and instituting adequate measures to effectively prevent/remove fogging, while not overreacting during low or cold ambient weather.
2. Description of Related Art
In modern vehicles it is common to have an HVAC system for a user compartment (hereinafter “cabin”). The HVAC system provides warm and cool air to the cabin of the vehicle and allows users to select a set temperature for the cabin. Once the set temperature is selected, the HVAC system will provide conditioned air to adjust the climate of the cabin to the set temperature. Further, the HVAC system is used to eliminate fog or condensation that appears on the vehicle windows.
In conventional automobile control systems, application of remedial measures to combat fogging of the vehicle windows is manually instituted by the vehicle passengers. Naturally, such remedial measures, which primarily consist of operating the HVAC system in a defrost mode, are employed after a fogging condition has developed, and typically require some time to alleviate the fogging condition. As will be appreciated, such systems are not suitable for automatic control, and do not anticipate impending fogging conditions.
In vehicles incorporating an automatic HVAC control system, internal and external conditions are sensed to determine whether fogging is likely. Such systems rely on knowing the window temperature and cabin conditions, namely cabin temperature and cabin relative humidity. After determining cabin temperature and cabin relative humidity, a dewpoint of the vehicle is known. In addition, the window temperature is determined by measurement or by estimation/calculation.
Then a fog margin, which is the difference between the cabin dewpoint and the measured or estimated/calculated glass temperature, is determined. The lower the value of the fog margin, the more likely that fogging of the window will occur. The fog margin is then used to control the HVAC system. Unfortunately, such known systems may overestimate the amount of air that is needed from the defroster in low or cold ambient temperatures to prevent/remove the fog. One of the reasons for this is the potential for error when the relative humidity is being measured during cold ambient air temperatures. Most automotive interior humidity sensors experience difficulty in accurately measuring the relative humidity at values less than 10% during these cold ambient air temperatures. Accordingly, the resultant dewpoint may be too large, thereby creating an artificially low fog margin. Thus, this artificially low fog margin can produce a false positive for fog presence thereby exposing the user to unnecessary noise from the fan and also excessive air currents from the fan that is operating at an increased speed to reduce/eliminate the nonexistent fog.
Therefore, there exists a need in the art for a method to better control the HVAC system so as to adequately detect and act upon possible fog conditions during low or cold ambient air temperatures.